1. Field of the Invention
This invention relates to an automatic precision lathe of a movable main spindle type for cutting a long workpiece such as a long shaft.
2. Description of the Related Art
Many shafts have an intermediate portion with the largest diameter. As the cutting operation of a workpiece of this type is continued on the conventional automatic lathe known as a Swiss type automatic lathe, the portion of the workpiece which is being cut has become remoter and remoter from the main spindle which holds the workpiece. If the workpiece is cut at the same rotational speed and at the same feed, the remoter portion which is being cut is bent more largely. In order to avoid this disadvantage, the remoter portion must be cut at a lower rotational speed and/or at a smaller feed, resulting in poor productivity. Further, the workpiece requires a long held portion in order to be securely held by the chunk of the main spindle and this portion must be cut off after the working of the workpiece is completed, with the result that a relative large length of material is wasted.
With the conventional automatic machine provided with workpiece holding means such as a workpiece supporting device or a subspindle in front of the main spindle, the means is adapted to pass the machined portion of the workpiece. Thus, the means must have a hole with an inner diameter equal to the outer diameter of the largest portion of the workpiece and the portions of the workpiece which have smaller diameters cannot be securely held by this holding means, thereby largely bending the workpiece bent during the cutting operation.